[Metallocene Compounds]
In recent years, metallocene compounds are well known as homogeneous catalysts for olefin polymerization. After the report of isotactic polymerization by W. Kaminsky et al. (see Non Patent Literature 1), many studies have been made on olefin polymerization, in particular, stereoregular α-olefin polymerization using metallocene compounds.
In α-olefin polymerization using metallocene compounds, it is known that the stereoregularity and the molecular weights of the obtainable α-olefin polymers are greatly varied by the introduction of substituents into the cyclopentadienyl ring ligands of the metallocene compounds or by the bridging of the two cyclopentadienyl rings.
[Bridged Metallocene Compounds]
For example, there are the following reports as to propylene polymerization catalyzed by metallocene compounds which have a ligand in which a cyclopentadienyl ring and a fluorenyl ring is bridged to each other.
From the viewpoint of stereoregularity, dimethylmethylene (cyclopentadienyl) (fluorenyl)zirconium dichloride affords syndiotactic polypropylene (see Non Patent Literature 2); dimethylmethylene(3-methylcyclopentadienyl) (fluorenyl) zirc onium dichloride having a methyl group at the 3-position of the cyclopentadienyl ring affords hemiisotactic polypropylene (see Patent Literature 1); and dimethylmethylene(3-tert-butylcyclopentadienyl) (fluorenyl) zirconium dichloride having a tert-butyl group at the 3-position of the cyclopentadienyl ring affords isotactic polypropylene (see Patent Literature 2).
Dimethylmethylene(3-tert-butyl-5-methylcyclopentadien yl) (3,6-di-tert-butylfluorenyl)zirconium dichloride having tert-butyl groups at the 3- and 6-positions of the fluorenyl ring affords polypropylene with higher isotactic stereoregularity than obtained with dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl) (f luorenyl)zirconium dichloride (see Patent Literature 3).
From the viewpoint of molecular weights, diphenylmethylene(cyclopentadienyl) (fluorenyl)zirconium dichloride having a cyclopentadienyl ring and a fluorenyl ring bridged via diphenylmethylene affords syndiotactic polypropylene having a higher molecular weight than obtained with dimethylmethylene (cyclopentadienyl) (fluorenyl)zirconium dichloride (see Patent Literature 4); diphenylmethylene(3-(2-adamantyl)-cyclopentadienyl) (fluorenyl)zirconium dichloride having a diphenylmethylene bridge affords isotactic-hemiisotactic polypropylene having a higher molecular weight than obtained with dimethylmethylene (3-(2-adamantyl)-cyclopentadienyl) (fluorenyl)zirconium dichloride (see Non Patent Literature 3); and dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl) (f luorenyl)zirconium dichloride having a methyl group at the 5-position of the cyclopentadienyl ring (the α-position relative to the bridge) affords isotactic polypropylene having a higher molecular weight than obtained with dimethylmethylene(3-tert-butylcyclopentadienyl) (fluorenyl) zirconium dichloride (see Patent Literature 5).
Further, dimethylmethylene(3-tert-butyl-2-methylcyclopentadienyl) (f luorenyl)zirconium dichloride and diphenylmethylene(3,4-dimethylcyclopentadienyl) (fluorenyl) zirconium dichloride having substituents at two adjacent positions on the cyclopentadienyl ring afford polypropylene having a lower molecular weight than obtained with dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl) (f luorenyl)zirconium dichloride and diphenylmethylene(3-methylcyclopentadienyl) (fluorenyl) zirc onium dichloride, respectively (see Patent Literatures 5 and 6).
[5-Membered Ring-Bridged Metallocene Compounds]
A study reports the polymerization of propylene catalyzed by a metallocene compound in which a cyclopentadienyl ring and a fluorenyl ring are bridged via a 5-membered ring. However, such metallocene compounds have low usefulness in industry because of the fact that the stereoregularity of the obtainable polypropylenes is very low (see Non Patent Literature 4).
A recent study reports a metallocene compound having a cyclopentadienyl ring and a fluorenyl ring bridged via a 5-membered ring which can afford polypropylene having relatively high stereoregularity (see Patent Literature 7).
These metallocene compounds mentioned above exhibit excellent polymerization performance. In some applications, however, the catalysts are often required to afford polymers having still higher stereoregularity or still higher molecular weight with higher economic efficiency, namely, with high catalytic activity even under high-temperature polymerization conditions. Improvements are thus required.
[Metallocene Compounds Having Substituted Indenyl Ligands]
According to reports, metallocene compounds having substituted indenyl ligands afford relatively high stereoregularity or molecular weight (see Patent Literatures 8 and 9). However, such compounds are unsatisfactory in terms of performance under economically efficient polymerization conditions.
Because metallocene compounds are soluble in reaction media, they are generally used to catalyze polymerization in the form of supported catalyst systems in slurry polymerization or gas phase polymerization. Specifically, the metallocene compounds are supported on solid carriers. However, it is known that the polymerization performances such as stereoregularity control of the aforementioned compounds are markedly decreased when they are used in the supported form on carriers as compared to in the absence of carriers.
[Metallocene Compounds Having Substituted Azulenyl Groups]
To solve such problems, for example, a recent study reports a metallocene compound having a substituted azulenyl group as a ligand (see Patent Literature 10). However, even such catalysts do not achieve sufficient performances such as stereoregularity control when the polymerization temperature is elevated to obtain economic efficiency or when the compounds are supported on solid carriers.
Under these circumstances, there have been demands for further improvements in the catalytic performances such as polymerization activity, stereoregularity control and molecular weight control of polymerization catalysts including metallocene compounds (hereinafter, also written as “metallocene catalysts”).
[Reports of Polymerization Using Metallocene Catalysts]
Regarding the polymerization of monomers other than propylene with use of metallocene catalysts, for example, there are studies reporting the polymerization of 1-butene catalyzed by a metallocene compound having an indene ring as a ligand (see Patent Literatures 11 and 12).
Further, the polymerization of 1-butene catalyzed by a metallocene compound having a fluorene ring as a ligand has been reported. Specifically, the polymerization using isopropylidene(3-t-butyl-5-methylcyclopentadienyl) (fluorenyl)zirconium dichloride has been disclosed (see Patent Literature 13). According to this report, the obtainable polybutene has lower regioirregularity due to 4,1-insertions and exhibits higher heat resistance and mechanical strength as compared to when the polymerization is catalyzed by a metallocene compound having an indene ring as a ligand. In some applications, however, the catalysts are often required to afford polymers having still higher stereoregularity or still higher molecular weight under more economically efficient conditions. Improvements are thus required.
A study reports the polymerization of 4-methyl-1-pentene as a main monomer (see Patent Literature 14). Patent Literature 14 describes that metallocene-catalyzed polymers outperform conventional Ziegler-Natta-catalyzed polymers in the balance of properties such as heat resistance and are thus highly useful in industry. However, applications sometimes require that the polymers have a still higher melting point/stereoregularity or have a still higher molecular weight.
In general, increasing the polymerization temperature enhances economic efficiency but at the same time tends to decrease the molecular weight or the melting point/stereoregularity of the obtainable polymers. Thus, there have been demands for the development of novel polymerization processes capable of producing polymers having a higher molecular weight and a higher melting point/higher stereoregularity.